I have a Deuter Race - it takes a 2l bladder. Comfy light weight pack that stays off the back; I have also owned a couple of camelbacks and an Ergon - I prefer the Deuter for its light weight and good fit.

Cheers

"A dessert without cheese is like a beautiful woman with only one eye."Jean Anthelme Brillat-Savarin

I have a deuter race. Hands down the most comfortable of all the back packs I own (its not the smallest either). I use it when I go MTB'ing.

For the roadie, if I use one, I'll use a camelbak like this one I have a 2l version one as well (not camelbak), the advantage of that 1.5l one is it is small enough that I can still get things out of my jersey pockets.

When the bladder is full its not crazy-comfortable as it bulges a bit, but drink a bit and it get better.